The present invention relates to an extruder for viscous masses, in particular thermoplastic melts and natural or synthetic rubber mixtures, and includes an extruder screw which is followed in the conveying direction, downstream, by a positive-displacement gear pump having at least one conveying gear and at least one positive-displacement gear.
In extruders of this type, it is known to arrange a gear pump, generally with two rotors, behind the extruder screw downstream in the conveying direction. The gear pump requires a separate drive and is flanged to the housing of the extruder screw, in which case narrowed portions may occur as a consequence of construction at the transition between the two housings.
The gear pumps are suitable for building high pressure gradients. They operate according to the volumetric conveyance principle in that the tooth cavities form closed-off chambers. Expulsion is virtually independent of the counterpressure and is proportional to the rotational speed. The gear pump dampens and equalizes the pressure pulsations, generated by the screw geometry, up to a factor of 10, thus resulting, in the same ratio, in improved tolerances in terms of weight and of dimension. Moreover, the efficiency of a gear pump is markedly higher, and therefore the mass to be conveyed can have a low temperature.
In known extruder arrangements, the gear pump is designed in the manner of an epicyclic or planetary gearing, the axes of the sun wheel, of the outer ring, of the planet carrier and of all the planet gears being arranged parallel to one another and to the axis of the extruder screw.
Epicyclic gear pumps, however, have the disadvantage that the filling pressure is relatively high due to the feed orifice and flow diversion.
It is therefore an object of the present invention to lower the filling pressure on the gear pump.